Artificial knee joint prostheses are widely used today, restoring mobility to patients affected by a variety of conditions, particularly arthritis. The satisfactory performance of these devices can be affected not only by the design of the prosthesis itself, but also by the final placement and spatial orientation of the implanted components both individually and together in combination. Improper placement or orientation of one or more components of the device or an improper fit to the patient's anatomy can adversely affect the goal of satisfactorily restoring the clinical biomechanics and function of the joint.
There are several general types of total knee joint replacement prostheses, including fixed bearing, mobile bearing, and rotating platform, in both hinged and semi-constrained designs. The primary difference between the fixed bearing designs and the mobile bearing and rotating platform designs is that the mobile bearing and rotating platform designs uncouple, to some degree, the motion of the tibial tray from the motion of the component that articulates with the femoral component. In a typical rotating platform design, for instance, the plastic articular component is free to rotate about an axis that is roughly parallel to the long axis of the tibia, while the tibial tray remains stationary relative to the tibia.
The positioning of the device, including the rotational alignment of the tibial component relative to the femoral component, affects the biomechanics of the joint. The proper rotational alignment of the tray on the tibia allows for greater coverage of the metaphyseal bone with more complete bony support of the tray and little or no overhang. The proper rotational alignment of the tibial articular surface with the femoral articular surface allows these components to articulate in the manner for which they were designed, and in this way maximizes the contact area between the articular surfaces, minimizes the contact stresses for a given flexion angle and joint load, and thus minimizes wear of the articular surfaces. While rotating platform and mobile bearing designs can provide this de-coupling of the tibial tray and articular surface alignments, these designs employ an additional articulation between the plastic tibial articular component and the supporting metal tray that is a potential source for material wear. It is therefore desirable to provide a component that de-couples the rotational alignment of the tray to the proximal tibia from the alignment of the tibial bearing component to the femoral component, thus allowing for both optimal articulation and secure engagement to be achieved, independent of each other, without the additional metal-to-plastic articulation that is incumbent to mobile bearing and rotating platform designs.